Re: Supercard

["Vince Briel" <vbriel@removethisspamcamcast.net>]

Reading the review, the device didn't perform as stated. Coffee lukewarm is 
what he calls it. POS, waste. Now, an externally powered warmer controlled 
via a timer on the Apple II would warm my coffee right up!

Vince

-- 
Making computers fun again
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http://www.brielcomputers.com

"Michael J. Mahon" <mjmahon@aol.com> wrote in message 
58idnd4oG4V2jhrZnZ2dnUVZ_rednZ2d@comcast.com">news:58idnd4oG4V2jhrZnZ2dnUVZ_rednZ2d@comcast.com...
> Mark McDougall wrote:
>> Henry wrote:
>>
>>> I just can't see how 5v will keep your cup warm though?  Traces on the 
>>> Motherboard of the system are probably a little warm too.
>>
>>
>> Yeah, interesting. They all claim to maintain a cup of coffee at 40-50 
>> degrees Celcius, but I can't get my 'paper napkin' calcs to work...
>>
>> Given that USB is limited to 500mA @5V = 2.5W.
>>
>> I based it on a 420ml coffee cup (already) at 40 deg C, and an ambient 
>> temperature of 20 deg C, assuming such a cup would take 30 mins to cool 
>> to ambient temperature. I also assumed a linear temperature gradient over 
>> the 30 mins (I believe the temperature gradient would actually *decrease* 
>> as it cooled).
>>
>> By my calcs, it would require 19J to be transferred to the liquid every 
>> second... (19W)... to *keep* it at 40 degs..
>>
>> .... out by a factor of 10!
>
> That sounds about right, if you are assuming (as pictured) that the
> cup is exposed to air on all surfaces except the bottom, and that the
> top of the liquid is also exposed to air.
>
> I wonder how the result would change if we assumed an insulated cup,
> with heat input at the bottom and heat loss only from the liquid
> surface on top?  (I'm sure that 2.5W is still insufficient to maintain
> a 20-degree C temperature differential.)
>
> Perhaps if the 2.5W were input to a thermoelectric heater, so that some
> heat was being pumped as well as dissipated resistively...  (I've
> actually seen one of these, that could be "reversed" to keep a drink
> cool[er] as well.)
>
> -michael
>
> Parallel computing for 8-bit Apple II's!
> Home page:  http://members.aol.com/MJMahon/
>
> "The wastebasket is our most important design
> tool--and it is seriously underused."